Arista ML2 mechanism driver clean up and integration with port binding

fixes bug: 1215097

This patch addresses minor cleanups and integration with port binding
and dhcp support into Arista ML2 mechanism driver.

Change-Id: Icebc4aa1c57278171cc6b5209107b8eab833249c
Merge "Arista ML2 mechanism driver clean up and integration with port binding"